Many types of visual display in current use rely on transmitted or reflected light to produce a visible image. Such displays are frequently provided with a “backlight” or “frontlight” which is a flat device, laid close and parallel to the actual display, that emits light into the display over its area. In many designs, the light is injected into one or more edges of the flat element of the backlight or frontlight, and the backlight or frontlight then acts a distributor.
There is currently a desire to produce thinner backlights and frontlights so that the devices that include and use these components can be as thin and lightweight as possible. This is particularly true for smartphones and tablets both of which make extensive use of such components. Further requirements for these components include: low cost; the ability be manufactured in high volume; and high optical efficiency. Although thin backlights (approximately 0.25 mm thick) have already been produced, there is then the problem of how to inject light from certain type of sources, such as light-emitting diodes (LEDs), into the “main” light-distribution and emission element of a backlight or frontlight. In particular, there is a problem of efficiently injecting light from such sources where the minimum dimension of the source is larger than the thickness of the “main” backlight or frontlight element. The minimum width of current LED sources is about 0.6 mm, which is over twice the thickness of the thinnest current thin backlights.
Several solutions to that problem are taught in U.S. Pat. No. 7,286,296 B2 and U.S. Pat. No. 7,755,838 B2 (by several of the same inventors as the present application), first published on Nov. 3, 2005 as US 2005/0243570 A, where there are optical embodiments that transform the shape of a source, such as a square, to another shape, such as a rectangle, but with the latter having a smaller dimension in one direction than before but with substantially the same area as the source. That prior art teaches using a combination of elements (luminance shifters and light guides) to create so-called étendue squeezers, which can, within certain limits, be used to inject substantially all the light from a source into a backlight when the minimum dimension of the source is larger than the thickness of the backlight. Additional information is found in Julio Chaves, Waqidi Falicoff, Oliver Dross, Juan Carlos Mitiano, Pablo Benitez, William A. Parkyn, “Combination of light sources and light distribution using manifold optics”, Proc. SPIE 6338, Nonimaging Optics and Efficient Illumination Systems III, 63380M (Sep. 11, 2006). In the interests of conciseness, information that is already known to persons skilled in the art through those and other publications is not unnecessarily repeated herein.
There are certain features of the approach that we taught in that prior art that are not always advantageous in every use. Firstly, the parts of those devices were envisioned to be made individually and then aligned and assembled together. They were not necessarily designed to be molded as one piece (even though one-piece molding may be possible using sophisticated molding techniques and in some assemblages more than one material). Secondly, the optical approach in that prior art conserves étendue while ensuring that the source area at the entry port is substantially the same at the exit port. Étendue conservation may not be the most appropriate approach if the goal is to fully flash a backlight of the solid dielectric type without having dark regions near the exit port or ports. The beam cone angle at the exit aperture of the étendue-squeezing optic into a solid dielectric backlight optic is limited by the critical angle related to the index of refraction of the material. It may be desirable to have a solid dielectric étendue squeezer which can be easily moldable as one part and which outputs a beam angle into a solid dielectric backlight optic that is equal to or wider in one direction than the one limited by the critical angle.
For some applications it would be desirable that the étendue-squeezed output has a larger area than the source at its exit to facilitate complete flashing of the backlight and to facilitate a reduction in the number of sources needed to fully flash the backlight. In other applications it would be desirable for the exit output area to remain the same or similar size to the source, but have the beam angle at the output port in the plane of the backlight main element (which in this specification is sometimes called the “horizontal” direction) increased. Further, it would be desirable if these embodiments are highly efficient and are easily moldable as one piece, or at least are designed for easy alignment and assembly for large volume production.